The Habitable Planet

Nebula

A giant cloud of gas and dust from which stars and planetary systems form (mostly hydrogen and helium; heavier atoms formed earlier in stars).

Nuclear fusion

The process where hydrogen nuclei fuse to form helium, releasing energy and powering stars.

Accretion disk

A flat disk of dust and gas around a young star where material accretes and planets begin to form.

Planetesimals

Tiny bodies formed when dust grains stick together; about a mile in diameter, seeds of planets.

Protoplanets

Early planets formed from planetesimals that grew by collisions and gravity.

Terrestrial planets

Rocky planets made of silicates, iron, and nickel; high melting points; rotate slowly; thin or no atmospheres.

Inner planets

Mercury, Venus, Earth, and Mars; small, rocky with thin atmospheres, solid surfaces, few or no moons, no rings.

Outer planets

Jupiter, Saturn, Uranus, and Neptune; giant with thick atmospheres, no solid surfaces, many rings and moons.

Jovian planets

Another term for the outer, gas-giant planets.

Dwarf planet

A smaller planet that cannot clear its orbit; five officially recognized: Ceres, Pluto, Haumea, Makemake, Eris.

Ceres

Dwarf planet located in the asteroid belt between Mars and Jupiter.

Pluto

Dwarf planet located in the Kuiper Belt beyond Neptune.

Haumea

Dwarf planet in the Kuiper Belt.

Makemake

Dwarf planet in the Kuiper Belt.

Eris

Dwarf planet in the Kuiper Belt.

Earth

The Blue Planet; the only known planet with liquid water on the surface; about 70% of the surface is water.

Blue Planet

Another name for Earth due to its large oceans.

Liquid water

Water in liquid form on Earth's surface; essential for life; covers roughly 70% of Earth.

Plate tectonics

Movement of Earth's lithospheric plates; shapes landforms and drives the carbon cycle.

Electromagnetic field

Earth's magnetic field that helps shield the planet and retain its atmosphere.

Mars

Fourth planet from the Sun; the Red Planet; cold desert world with a thin atmosphere and features like volcanoes, canyons, and polar ice caps.

Terraforming Mars

The idea of altering Mars' environment to be more Earth-like for human habitation.

Asteroid belt

Region between Mars and Jupiter filled with many asteroids.

Kuiper Belt

Region beyond Neptune containing many icy bodies and dwarf planets.

Sun

The star at the center of the Solar System; formed from a nebula and powered by nuclear fusion.

Solar System

The Sun, eight planets and their moons, the asteroid belt, comets, and other bodies.

Formation age of Solar System

Formed about 4.6 billion years ago from a rotating cloud of gas and dust.

Age after Big Bang

The Sun and Solar System formed about 9.2 billion years after the Big Bang.